In such a board, the multilayer network is ordinarily supported by a substrate made of an insulating wafer incorporating the distribution planes of the supply potentials of the integrated circuit or circuits. The substrate may be a wafer of fired ceramic, or a wafer made of an organic material, such as plastic. The board may also be simply made of the multilayer network, in which case it takes the form of a flexible board. Then again, the board may be a wafer of semiconductor material incorporating the integrated circuits and covered with a multilayer network for the interconnection of these integrated circuits by the technology known as wafer scale integration, or WSI. In all these boards, the multilayer network is composed of a stack of alternating conductive and insulating layers. In this stack, an insulating layer is pierced with via holes for the electrical connection of the conductive layers with one another. Ordinarily, the conductive layers are made of aluminum or copper, and the insulating layers are presently preferably made of a polymerized material, such as a polyimide.
One problem in the manufacture of the multilayer metal network lies in optimizing the density of the multilayer metal network. The via holes are currently flared, to facilitate the deposition of the upper metal layer. However, the flaring requires precise positioning on each via hole and also requires control of the degree of flaring. Moreover, the flaring necessitates enlarging the conductors, to the detriment of the large scale of integration sought for the conductors in the network. To solve this problem, the via holes are simply cylindrical or prismatic holes. However, the presence of these simple holes presents a second problem, which is to obtain relatively plane layers in order to superimpose a large number of metal layers on one another in the network. This problem is increasingly serious, the thicker the insulating layers. In current boards, the insulating layers may have a thickness of more than 10 micrometers. One solution to the problem is to fill the via holes, at least partially, with a metal material. The filling results in what is currently known as a via. Because of the vias, the deformation in the upper conductive layers is reduced.
A known method of via manufacture comprises chemical (electroless) nickel deposition. The via hole is first put in contact with a catalytic solution, generally based on palladium, to obtain an activated metal surface. The via hole is then plunged into a bath based on dimethyl aminoborane, known by the abbreviation DMAB, which serves as a reducing agent. Polyimides, in particular, prove to be inert to DMAB. This method of chemical deposition of nickel can be used on any activated metal surface.
The best known description of the phenomenon of chemical (electroless) deposition is that by Van Meerakker in an article in the Journal of Applied Electrochemistry, 1981, Vol. 11, pp. 395 ff. The deposition of metal proceeds with the simultaneous reactions of anodic oxidation of the reducing agent and cathodic reduction of the metal. The activation of the metal surface of the substrate that forms the base of a via hole must permit both dehydrogenation of the reducer and desorption of the hydrogen by recombination or oxidation, at a speed sufficient for the metal deposition to occur. The activation is done by treating the metal surface with a catalytic solution, generally made on the basis of palladium. For example, activation done by a moist method comprises putting the insulating layer and its via holes into contact with a solution of palladium chloride (PdCl2). In this case, the activation theoretically takes place only on the metal surface at the bottom of each via hole. In the following DMAB bath, the palladium present at the bottom of the via holes collects the nickel. Normally, the surface of the insulating layer is inert to this type of bath. However, experience has shown that nickel nucleation appears on the surface of the insulating layer that has undergone activation. This nucleation creates insulation defects in the finished multilayer network. To avoid these defects, the outer surface of the insulating layer must be protected with a mask that allows only the bottom of the via holes to show.
Another form of catalytic treatment is the conventional method known as the "lift-off" method. This method comprising forming a photoresist mask on the insulating layer in such a way that only the via holes show, then evaporating the palladium in a vacuum. The palladium uniformly covers the bottom of the via holes and their lateral walls as well as the entire surface of the mask. The mask is then eliminated. After the catalytic treatment, the board is plunged into a DMAB bath for chemical deposition of nickel. This deposition forms in the presence of palladium, beginning at all the walls of each via hole.
The two forms of catalytic processing have disadvantages. In the first form described above, the insulating layer must be covered with a photoresist mask that allows only the via holes to show, to avoid nickel nucleation. As for the lift-off method, the mask is normally necessary. The masking method requires several operations, necessitating precise centering on the via holes. Hence the masking is delicate, tedious and expensive. Moreover, it presents a serious problem when the via holes are deep, as is generally the case with interconnection boards. The masking begins with spreading of a photoresist resin, for example by rotating the board. The resin layer fills the via holes and has a relatively plane surface above the insulating layer. In other words, the resin layer has a much greater thickness inside the via holes than outside them. The conditions of insulation of a positive photosensitive resin, or of developing a negative photosensitive resin depend on the thickness of the resin. Hence it proves difficult to obtain a mask limited only to the contour of the via holes and leaving no trace of resin whatever on the walls of the via hole. Furthermore, this operation requires precise centering over each hole. In summary, the masking that is necessary in the conventional methods comprises supplementary tedious, delicate and expensive operations, and does not give satisfactory results.
The "lift-off" method has another disadvantage. Because of the presence of palladium on all the walls of a via hole, the growth of nickel in the via hole proceeds both vertically and laterally. When the via hole has a diameter greater than its depth, the layer of nickel grows in the form of a U and has an indentation. Furthermore, the upper edges of the layer of nickel are raised and form a ring above the surface of the insulating layer. Both the indentation and the ring conflict with the desired leveling.
Additionally, the known methods of manufacture of multilayer metal networks comprise iterative formation of the complete layers desired. An insulating layer is formed over a metal layer and etched to make the via holes there. Next, the via holes are filled to make the vias, and the insulating layer and its vias are covered with another metal layer. These steps are repeated for the upper layers. To increase the scale of integration of the interconnection of the network, it is desirable to superimpose the vias of the network. This is possible if the vias are coplanar with the insulating layer in which they are incorporated. The moist process of chemical deposition of nickel produces relatively coplanar vias. However, each via has an edge, because there is an edge effect in the growth of the nickel in contact with the lateral walls of the via hole. This edge creates a slight indentation on the upper surface of the via. The upper via in a stack will have a more pronounced indentation. Excessively great difference in level, in other words unevenness, has the disadvantage of creating defects in the metal layer. Consequently, in practice it is impossible to superimpose more than two or three thus-formed vias. The superimposition requires very precise centering in all the operations of masking and etching, in order for two vias to be aligned correctly. Moreover, these operations are necessary in the known methods. The method of "lift-off" chemical deposition creates a major depression in wide via holes and makes it practically impossible to form an upper via. The situation is the same for the flared via holes. Currently, the solution used is to stagger the upper via holes. This staggering is often done in a spiral about a vertical line. However, the cross section of this spiral is much greater than that of the via holes, which conflicts with the desired large scale of integration. Furthermore, a spiral having via holes 90.degree. away from one another, for example, means that the fifth via hole will be superimposed on the bottom one and will create an excessive depression. The cross section of the spiral must accordingly be increased, to the detriment of the large scale of integration sought.